Secret-communication system



B. P. HAMILTON.

SECRET COMMUNICATION SYSTEM.

APPLICATION FILED DEC. 27 I919.

Patented. Mar. 1, 1921.

INVENTOR.

f ATTORNEYS.

PATENT err-ice.

UNI ED STATES BAX'IEB P. HAMILTON, O3 BROOKLYN, NEW $03K. ASSIGNOB .lO AMERICAN TELE- PHONE AND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK.

Specification of letters Patcnt.

Application filed December 27, 1919. Serial No. 847,717.

It is a general'purpose of the invention to provide a system of telephonic communication wherein secrecy may be assured 111 the transmission of messages between distant stations so that unauthorized persons who may have access to the medium through or over which the messages are transmitted will not be able to obtain information from such messages. The secrecy arrangements of this invention are applica ale to both radio telephone systems and to telephone systems employing metallic conductors as the medium of transmission. Accordingly the arrangements of the invention provide means for reventing the possibility of information iieing obtained from a telephone system either by tapping the wires by direct metallic connection or inductive connection or 80 by being tuned to the frequency of the sending station in a radio system. Other features and pur oses of the invention wlll appear more fu ly tion hereinafter given.

In the arran ements of the invention there is associate with the transmitting circuit atthe sendin station a-plurality of tuned shunts, sac of said shunts being tuned to be resonant'to a particular narrow 0 band of the frequencies of the telephonic current. A sufiicient tuned shunts may be provided to practically cover the voice range of frequencies. These tuned shunts are controlled by a selecting 46 device operating in a eontinuouslly varyin manner m accordance with a'pre etermme plan or scheme. With this arrangement certain bands of the frequencies of the talking current may be selected and these bands may 50 be amplified. and then impressed on the outgoing line. Accordingly certain frequencies of the outgoing telep amplified while others may not be amplified or certain bands ma be amplified more than 56 others and the tal ing current applied to tuned to be resonant to a particular band 111! from the detailed descripnumber of differently onic currents" may be" the outgoing line will accordingly be distorted and rendered unintelligible. At the receiving station there is rovided a plurality of tuned shunts or ab which operate to attenuate the frequency bands w ich were am lified and bring the am litudes thereof bac to their original relation to the other frequencies and so render the voice currents intelligible in a receiver. These tuned shunts or absorbing devices are controlled at the receiving station by a selecting device operating in synchronism with the selecting device at the sending station and operating in accordance with the same predetermined plan or scheme as said device. Accordingly by preserving secrecy as to the lan or scheme of selecting and amplifying t e frequency bands at the sending station, or by constantly changing the plan or scheme therefor at both sta- Patented Mar. 1, 1921.

sorbing devices I tions, it will be practically impossible for one who has unauthorized access to the medium through or over which the messa es are transmitted to obtain the information contained in such messages.

The invention may be more fully understood from the following description. taken in connection with the accom anying drawing in the figure of which is ill cuit diagram embodyingthe arran ments of the invention as ap lied to a tel phone system employing meta lic conductors as the conducting medium. A

In the drawing is shown a transmission line L interconnecting two stations A and B. The sending apparatus at. station A is illustrated in full and the receiving apparatus at station B is shown in full, although each station will include both sending and re-. ceiving apparatus. The sending circuit 3 at station A is connected to line L by being bridged across the midpoints of the windings of the three winding'transformer 1.

Inductively related to line L at station A ustrated a cit-- tuned shunts such as 10 and 11 and other shunts not shown. Each of these shunts is tire range of voice requencies.

of frequencies within the voice range and a sufficient number of differently tuned shunts may be provided to ractically cover the en- Associated with each of the tuned shunts, such as shunts 10 and 11, are circuits, such as 15 and 16, which circuits are associated with the sendin circuit 3 through transformers and amplifiers, such as amplifiers A and A The amplifiers A and A may be adjusted to give a (greater do of amplification than am li er A,, an --furthermore the amplifiers and A may even be adjusted to ive different degrees of amplification as iietween themselves. Accordingly the frequency bands selected by the tuned shunts will be amplified to a greater degree than the other telephonic frequencies from the transmitter 14 and the electromagnetic waves transmitted out over line L will be considerably distorted and will be rendered unintelligible. The tuned shunts, such as 10 and 11, are controlled by switches, such as 21 and 22, included in the printing telegraph receiver 23. The printing telegraph receiver 23 may be of the type illustrated in the U. S. patent to Yorke, No. 1,215,604, issued Feb. 13,1917, in which a plurality of selecting bars control the printing of characters. In the device 23 t e selecting bars may be utilized to control switching mechanisms, such as the switches 21 and 22 and enou h selecting bars may be provided to contro any desired number of switches. The rinting telegraph receiver 23 is associated with the usual type of printing teleph transmitter 25 by the conductors 24.

he conductors 24 are controlled in the usual manner by the customary set of contacts in the printing telegraph transmitter, which contacts are in turn controlled by the perforated tape 26. The perforated tape 26 1s moved along step by step by the stepping magnet 27 which inturn is controlled by the circuit 28. Circuit 28 includes a. generator 30v generating currents of a frequency outside of the voice range and which for pur-.

pose of illustration may be assumed to be of approximately 100 cycles. Circuit 28 is controlled by the distributer 29, the brush of which will come inccontact With the segment thereof at certain intervals and momentarily close circuit 28. Every time .cir-

cuit 28 is closed the magnet 27 will operate row frequency band as its corres of the three winding transformer 2 is a circuit 5 with which may be associated sending apparatus similar to that already described with respect to station A. The receiving circuit 6 is associated with line L through transformer 2 and includes receiv ing a paratus such as the telephone receiver 34. iridged across circuit 6 is a plurality of tuned shunts, such as 32 and 33 and other tuned shunts not shown. One of these tuned shunts is provided across circuit 6 for each of the tuned shunts across circuit 9 and is tuned to be resonant to the same narnding tuned shunt across circuit 9. Furt ermore the impedance of the tuned shunts, such as 32 and 33, is so adjusted that the admittance characteristics of each shunt will serve to attenuate the amplified frequencies transmitted thereover so as to restore the amplitudes thereofto their original relation to the amplitudes of the other voice frequencies and accordingly remove the distortion and render such' voice currents intelligible in the receiver 34. The tuned shunts across circuit 6 are controlled by switches, such as 36 and 37, included in a printing telegraph receiver 38, similar to the device 23 already described with respect to station A. The

rintin telegraph receiver 38 is controlled )y con uctors 39 and the contacts of a well known ty e of printing telegraph transmit ter 40 in t e usual well known manner. The contacts of transmitter 40 are controlled by a perforated tape .41 which is indentical to tape 26 at the sending station. Tape 41 is moved along step by ste by the stepping magnet 42 which is contro led by the circuit 43. Circuit 43 is inductively related to'a tuned shunt 35 across circuit 6. Shunt 35 is tuned to be resonant to the frequency of the cm'rentsproduced by generator 30 and which for purposes of illustration have been assumed to b01100 cycle currents.

The operation of the arrangements of the 110 invention is as follows. When it is desired to talk from station A to station B, the tape 26 will have been inserted in the printing telegraph transmitter 25 and its perforations will have caused the contacts of the transmitter to have assumed a certain arrangement and thereby operated device 23 in an arbitrary manner to close certain of the switches therein. For purposes of illustration let it be assumed that, when talk starts, switch 22 has been operated and the shunt l1 closed thereby. The voice currents will be transmitted from the transmitter 14, over circuit 9, through transformer 8, through amplifier A,, which is adjusted to give low gain, through transformer 7 over circuit 3, and out over line L. The shunt 11 however is tuned to be resonant to a certain narrow band of frequencies within the frequency range of these voice currents. Accordingly I the portion of the voice currents within this frequency band will be transmitted over shunt 11', through transformer 13, circuit 16, transformer 17 through amplifier A transformer 20, circuit 3, and thence over line L. The amplifier A may be adjusted to 'give. a

eater de es of amplification than amplier A an accordingly the frequencies of the currents within the narrow fre uency band will be amplified to a greater egree than the other frequencies of the voice currents and accordingly the currents transmitted out over line L will be distorted and rendered unintelligible; These currents in distorted'form will be transmitted from line L through transformer 2 and over circu1t 6 at station B. It is pointed out that the tape 41, which is identical to the tape 26 will have been inserted in the telegraph transmitter 40 in the same relative position with respect to its perforations as tape 26 and accordingly the contacts of transmitter 40 Will have assumed an arrangement similar to the contacts of transmitter 25 and will have caused the device 38 to have operated to control certain of its switching mechanism in a similar manner to device 23. Let 1t be assumed that the device 38 has closed switch 36 and thereby closed. shunt 32. Shunt 32 is tuned to be resonant to frequencies within the same band as shunt 11. Accordingly that portion of the incoming voice currents of frequencies within said band and which was amplified out of proportion to the other frequencies will be transmitted over shunt 32. The impedance of shunt 32 1s so adjusted that the admittance characteristics of the shunt will serve to sufliciently attenuate these amplified frequencies to restore them to their original amplitude relation to the other frequencies of the volce currents and accordingly when the voice currents are transmitted through receiver 34 the distortion will have been removed and the voice currents rendered intelligible. As the talk continues from station A the distributor 29 will at intervals close the circuit 28 and 0perate the step ing magnet 27 and move tape 26 along anot er step. The new set of perforations in tape 26 w1ll cause a new arrangement of the contacts of transmitter 25 and a new operation of device 23 and cause a different one of the tuned shunts across circuit 9 to be closed. Accordin ly a different frequency band will be se ectedand impressed on the line in amplified form and the voice currents distorted in a different manner. However the closing of circuit 28 will also cause a 100 cycle impulse to be transmitted through transformer 31, over circuit 3, and out over line L. This 100 cycle impulse willbe transmitted over circuit 6 and through the shunt 35 which 1s tuned to be resonant to this frequency and thence will be transmitted over circuit 43 and will operate the stepping magnet 42 simultaneously with the aforementioned steppin magnet 27. The magnet 42 will move t e tape 41 along a step and thus chan e the. arrangement ,of contacts and switc es in devices 40 and 38 in an identical manner to devices 25 and 23. Accordingly every time a new tuned shunt is closed across circuit 9 and the manner of distorting the outgoing voice currents is thereby varied, a corresponding new tuned shunt will be closed across circuit 6 for removing the distortion. With this arrangement the outgoing voice currents may be distorted in a manner continuously varying in accordance with a redetermined plan or scheme, as determined by the selection of a key tape.

Accordingly by maintaining secrecy as to the key tapes utilized or by frequentl changing the identlcal key tapes, at each station, 1t will be impossible for one who has unauthorized access to the transmitting medium to obtain information from the messages transmitted therethrough or thereover.

While the arrangements of the invention have been illustrated in a transmission systom employing metallic conductors as a transmitting medium, it is understood that they may equally well be employed in a radio system. Furthermore while the nature of the message currents has been referred to as telephonic, it is understood that the arrangements of the invention are applicable to systems in which the message carrying currents might be of a nature other than telephonic. While tuned circuits have been referred to as the selective means for picking out the various frequency bands, other se ective devices, such as band filters of the types illustrated in the U. S. Patents #1227413 and #1,227,114 to G. A. Campbell may equally well be used. Accordingly while the arrangements of the .inventlon have been disclosed as embodied in certain specific arrangements which are deemed desirable it is understood that they are capable A of embodiment in many and widely varied forms without departing from the spiritof the invention as defined in the appended claims.

What is claimed is:

1. In a transmission system including a sending station and a receiving station the method of maintaining secrecy 1n the transmission of messages between said stations which consists 1n distorting the outgoing message currents by subjecting certain of the frequency bands of said currents to unequal degrees of amplification, and in restoring the amplitude relation of the several frequency bands of said message currents to normal at the receiving station bysubjecting certain of the frequency bands of the incoming message currents to unequal degrees of attenuation.

2. A transmission system including a sending station and a receiving station, a transmitting circuit at said sending station, means for selecting certain of the frequency bands transmitted over said circuit, means for amplifying said selected frequency bands to a greater degree than the other of said frequency bands, means for transmitting all of said frequency bands to said receiving station, means at said receiving station for attenuating said selected frequency bands to a greater degree than the other fre-. quency bands.

3. A transmission system including a sending station and a receiving station, a transmitting circuit at said sending station, means for selecting certain of the frequency bands transmitted over said circuit, means for amplifying said selected frequency bands to a greater degree than the other of said frequency bands, means for transmitting all of said frequency bands to said receiving station, means at said receiving station for attenuating said selected frequency bands to a greater degree than the other frequency bands, said first mentioned means and said last mentioned means operating in a variable manner and in accordance with a predetermined plan.

4. A transmission system including a 1,see,soe

sending station and a receiving station, an outgoing circuit at said sending station, a telephone circuit associated with said outgoing circuit, a plurality of normally, open shunts across said telephone circuit,' each of said shunts being tuned to be resonant to certain diil'erent bands of the frequencies transmitted over said telephone circuit, a local circuit associated with each of said shunts and with said outgoing circuit, am-

plifying means in each of said local circuits,

means operating in a variable manner and in accordance with a predetermined plan for selecting and closing certain of said shunts, an incoming circuit at said receiving station, a plurality of normally open shunts across said incoming circuit, each of said shunts being tuned to be resonant to a frequency band corresponding to the frequency bands to which said shunts across said telephone circuit are tuned, and each of said shunts having admittance characterstics sufficient to counteract said amplifying means in said local circuits, and means for selecting and closing said last mentioned tuned shunts, said means operating in synchronism and in accordance with the same variable and predetermined plans as the selecting means at said sending station.

In testimony whereof, I have signed my name to this specification this 24th day of December 1919.

BAXT'ER P. HAMILTON. 

